Most electronic devices generate heat during normal operation. Electronic devices can malfunction and fail if they overheat, and therefore manufacturers endeavor to design the devices so that overheating does not occur. Manufacturers of electronic devices incorporating semiconductors typically design each electronic device with a maximum allowable junction temperature at which the electronic device should function correctly. The junction temperature of a semiconductor device is the temperature of the semiconductor junction where the semiconductor device generates a maximum amount of heat. Increasing junction temperature can reduce product lifetime of the electronic device and increase the likelihood of product failure. Therefore, a cautious manufacturer will usually design the electronic device to operate below the maximum allowable junction temperature at a maximum specified ambient temperature to increase lifetime and reliability, where the ambient temperature is the air temperature of the surrounding environment.
For example, consumer electronics products may be required to operate at maximum junction temperature of 125° C. and maximum ambient temperatures up to 45° C., while commercial and automotive electronics are typically required to operate at a maximum junction temperature of 125° C., but ambient temperatures up to 70° C. and 85° C., respectively. The manufacturer may also specify a minimum ambient temperature for the electronic device.
If the maximum junction temperature of a semiconductor device is 125° C. and the maximum ambient temperature is specified at 45° C., then the device must generate no more than 80° C. at the semiconductor junction. During operation of an electronic device, the ambient temperature may vary, but the electronic device dissipates heat in a manner such that the junction temperature always remains less than some A above the ambient temperature. Given this information, the manufacturer typically calculates how much power can be input to the semiconductor device to keep the junction temperature within this limit. The manufacturer may also design sufficient cooling of the device to enable it to dissipate the junction temperature heat and operate safely at the maximum ambient temperature, whether by convection into the air or with the aid of cooling devices. After the design stage, electronic devices are manufactured and shipped to the consumer.
During operation, electronic devices having variable power inputs may reduce heat at elevated temperatures by limiting the power input to the semiconductor devices, but doing so also results in reduced performance. Electronic devices having fixed power inputs typically reduce heat at elevated temperatures by increasing cooling through active external cooling devices, such as by increasing the speed of a fan. Examples of such devices include certain laptop computers. The active cooling devices are typically run as a function of the clock frequency of the device, ambient temperature, and/or junction temperature. For example, an electronic device may run the fan at the highest speed when the clock frequency of the CPU is maximized or when a device is at or near the maximum ambient or junction temperature, and then lower the fan speed at reduced clock speed and/or lower ambient/junction temperatures to save power and reduce noise.
An example of an electronic device is a display device. A specific type of display device is a video projector, which takes a video signal and projects a corresponding image on a screen or other flat surface using a lens and an illumination source. When light emitting diodes (LEDs) or solid state lasers are used for the illumination source, the drive current input to the devices directly affects the brightness of the display. In some applications, a projector may use a combination of solid state lasers and LEDs. For come colors one or the other is more economical. However, increasing the drive current increases the brightness of the devices, but also increases the junction temperature, which reduces the life of the devices and, potentially, other components of the video projector.
Traditional LED and solid state laser projectors are designed the same way as electronic devices in general. Manufacturers design them to operate at a maximum junction temperature and at minimum and maximum ambient temperatures. The manufacturers then determine how much power or drive current to input to the illumination source during operation, and provide the projectors with a cooling device, such as a fan, that is run at a sufficient speed to prevent the projector, and in particular the illumination device from overheating. If the ambient or junction temperature decreases during operation, the cooling device may be run at a lower speed to reduce noise and conserve power.
Thus, conventional electronic devices, such as LED projectors, use a fixed drive current and variable cooling to keep the devices operating at or below the specified maximum junction temperature, which rises and falls based on the ambient temperature. Although such a design methodology keeps devices operating at below the specified maximum junction temperature and may maximize the lifetime of the devices, such techniques fail to maximize performance at all ambient temperatures.
For example, assume that the maximum reliable junction temperature of an LED projector is 125° C. and the maximum ambient temperature is specified at 45° C. In this case, the LEDs will operate at 80° C. above ambient temperature and assume the LEDs produce 50 lm of brightness. If during operation of the LED projector, the ambient temperature falls to 35° C. or to 25° C., then the junction temperature of the LED may also reduce to 115° C. and 105° C., respectively. In each instance, however, the LED will continue to operate approximately at a junction temperature of about 80° C. above ambient, and the brightness of LED remains unchanged at 50 lm. This example may be oversimplified in that as the ambient temperature decreases, the junction temperature must follow, but perhaps not exactly linearly, as the cooling settings may be reduced as ambient temperature decreases. Additionally, reducing the junction temperature can increase the efficiency of the LED so the LED brightness may increase a few lumens as the junction temperature decreases.
Generally speaking, while decreasing the performance of cooling devices in electronic devices results in increased heating of the devices and may result in decreased performance, increasing the performance of the cooling device results in more cooling, but does not increase device performance. This is because the maximum junction temperature is used to calculate a constant input drive current, which in turn places a limitation on the output brightness of the LEDs. The result in the case of LED projectors is that whether the ambient temperature falls and/or whether the cooling device is operated at maximum, the LED produces substantially the same brightness at lower ambient temperatures as it does at higher ambient temperatures.